EP3351951B1 - Method and devices for detecting an interruption of a protective earth connection by means of a leakage current spectrum - Google Patents

Description

The invention relates to a method and an electrical protection device for detecting an interruption of a protective conductor connection to a subsystem of a grounded power supply system, wherein an inverter system is connected to the subsystem.

In a power supply system, protective earthing of conductive tangible parts of an electrical equipment as a protective measure is an important part of the normatively required protective measure "protection by automatic disconnection of the power supply". This applies regardless of whether it is the network form of an ungrounded power supply system (French Isolé Terre - IT network) or a grounded power supply system (French Terre Neutre - TN network or French Terre Terre - TT network).

Protective grounding of a subsystem of a branched power system through a protective conductor connection to the subsystem therefore deserves special attention, since in most cases this protection is rendered ineffective by interruption of the protective conductor.

Under subsystem here is a turn-off unit of a total power supply system understood. This subsystem typically includes one or more electrical resources.

If another (second) fault is added to the interruption of one protective conductor connection (first fault), such as the failure of the basic insulation by bridging clearances and creepage distances or due to faulty insulation, there is an increased risk of electric shock.

Since the occurrence of this two-fault situation in power supply systems is not negligible, the use of residual-current-operated protective devices (RCDs) has gained ground in grounded power supply systems as additional protection.

In many industrial power supply systems, however, the use of RCDs as additional protection against electric shock is not possible, since, for example, even without an additional fault current, a leakage current already flowing through the power supply system which can well exceed 30 mA and thus in the Power supply system would cause existing RCDs to trip immediately.

If the RCD can not be used in a grounded power supply system due to excessively high leakage currents or if the RCD used is not suitable for protection against electric shock (designed only for fire and system protection), there is a danger of an interrupted protective conductor connection and a second fault in that a person, when the equipment is used as intended, suffers a dangerous electrical accident, as the fault circuit closes over the person's body.

In the case of an intact protective conductor connection, on the other hand, if the basic insulation fails, the residual current flows almost exclusively via the protective conductor back to the feeding point of the grounded power supply system. However, this leads - with correct design of the grounded power supply system - to very high ground fault currents and usually also to touch voltages with dangerously high amplitude. For this reason, a grounded power supply system must be shut down fast enough at a first fault.

Special attention should be paid to the installation of converter systems in grounded power systems. The protective conductor connection to a converter system is particularly critical, since insulation faults at the output of the converter system against touchable and conductive parts of a converter-controlled drive can lead to fault currents, in addition to mains frequency shares also a fairly broad spectrum of converter-specific spectral components of DC components up to shares in MHz Range.

It should also be noted that large leakage capacitances between the output phases of the converter and the drive housing (output filter) for the higher-frequency components can represent a low-impedance connection.

Conventional type A RCDs do not provide reliable additional protection here. A contact between the converter-controlled drive and an incorrect protective conductor connection can cause an electric shock without an RCD type A recognizing this. Even the use of mixed-frequency sensitive RCDs of type F usually does not help reliably against the risk of electric shock.

In terms of operation, the leakage currents in the switching frequency range of the inverter (kHz range) are usually well above 30 mA; in high-performance converter drives, the leakage current limit of 300 mA is often exceeded during operation. The use of an RCD is not possible in such systems, even for reasons of fire safety.

One of the most important protective measures, especially in high-performance converter drives, is thus a reliable protective earthing of the accessible conductive drive parts.

Even in an unearthed power system where, by definition, all active parts of the power system are disconnected from earth potential - ground and the connected equipment is grounded to a grounding system through a protective grounding conductor, a two fault situation can be dangerous if a piece of equipment is touched wide unearthed power system with a consequently large total network leakage capacity. The fault circuit is closed in this two-fault situation on the touching person and the network leakage capacity.

If the protective conductor connection is intact, the fault current in the case of faulty basic insulation flows almost completely across the protective conductor and the system leakage capacitances. Even in the case of a simple fault in an unearthed power supply system, this only leads to harmless contact voltages on the equipment. For this reason, an ungrounded power supply system can continue operating on a first fault.

In order to counteract the risk arising from an interrupted protective conductor, solutions are known from the prior art which, however, sometimes have considerable disadvantages.

From the publication WO 03/100938 A1 is a residual current circuit breaker with a spectral characterization based signal analysis of the fault current known. In particular, with not purely sinusoidal fault currents so that a predictable tripping behavior can be achieved. Power work points of an inverter system are not taken into account.

The publication EP 2568557 A1 shows a method for detecting fault currents for a residual-current circuit breaker in inverter operation. In this case, for more reliable evaluation of the frequency spectrum of the leakage current switching information about switching operations of the frequency converter to the residual current circuit breaker required.

The publication EP 2568560 A1 describes a method for detecting and optionally blocking a fault current in a frequency converter, wherein the time course or a spectral band of a current flowing through the frequency converter current is evaluated.

Further proposals exist for a selective fault current detection, which can distinguish between leakage currents and fault currents. However, RCDs based on these ideas are not available because the reliable function in the 3-phase AC system has not been proven so far.

Furthermore, devices are available on the market, which are to enable the use of residual current protective devices for protection against electric shock in industrial systems by compensating for capacitive leakage currents. However, it is not known how reliably such protective devices work in widely branched industrial networks with changing, complex operating states.

Finally, there are loop monitoring devices on the market, which make a protective conductor connection monitoring directly to the resources. With a variety of resources at different network branches a corresponding number of loop monitoring devices are required.

The present invention is therefore based on the object to provide a method and an electrical protective device, in grounded branched, ie provided with turn-off units (subsystems), power supply systems interruption of a Detecting the protective earth connection to a subsystem beforehand without interrupting the operation. Of particular importance in a grounded power supply system is the special case of an inverter system connected to the subsystem.

For a grounded power supply system to the subsystem of which an inverter system is connected, the object is achieved by a method according to the invention as claimed in claim 1.

The starting point in this constellation is a measurement of a leakage current specific for the converter system in a fault-free state of the grounded power supply system. This measurement is made for all relevant, operationally expected power operating points of the converter system.

A transformation of the time profile of the respective leakage current in the spectral range serves as reference for this inverter system reference leakage current spectrum and shows a characteristic of a particular power operating point of the inverter expression with intact protective conductor connection.

An interruption of the protective conductor connection to the converter system connected to the subsystem causes a omission of the subsystem discharge capacity of the subsystem and, as a result, a detectable change in the leakage current spectra.

During operation of the power supply system with the inverter system connected via the subsystem, the current leakage current is continuously measured and from this a current leakage current spectrum corresponding to the instantaneous power working point of the converter is calculated. This leakage current spectrum is compared with the corresponding reference leakage current spectrum, wherein a deviation between the currently calculated leakage current spectrum and the corresponding reference leakage current spectrum indicates an interruption of the protective conductor connection.

In a further embodiment, the calculation of the converter system-specific reference leakage current spectrum and the calculation of the current leakage current spectrum during operation of the converter system a calculation of significant leakage spectral components, wherein the significant leakage spectral components are assigned to the power work points during commissioning of the power conversion system in a learning phase.

With regard to a reduction of the computational effort in the calculation of the leakage current spectra, the calculation of certain significant spectral components within the entire leakage current spectrum is sufficient. The significance of the selected leakage current spectral components results from the fact that a clear (amplitude) change is recognizable at these frequency locations as a result of the omission of the subsystem discharge capacity of the subsystem.

In a learning phase during commissioning of the converter system, the assignment of the significant leakage current spectral components to the power work points takes place.

In an advantageous manner, the testing of the currently calculated leakage current spectra is carried out in each case by a comparison at locations of the significant leakage current spectral components.

In order to check whether the currently calculated leakage current spectrum has a deviation from the corresponding reference leakage current spectrum, it is thus not necessary to calculate the total leakage current spectrum, but the test is carried out with little computational effort on the basis of the selected significant leakage current spectral components.

Advantageously, the reference values Referenz-Gesamtnetzableitkapazität, Referenz-Gesamtdifferenzstrom, Reference-Ableitstromspektrum at a first commissioning and immediately after a retest of the power system takes place.

During a first commissioning and immediately after a repeat test, it can be assumed that the power supply system, in particular the protective conductor connections, is in faultless condition Condition is / are located, so that reliable reference values can be determined in this phase.

As an alternative or in addition to the determination of the reference values during the initial startup and the repeat test, the determination of the reference values reference total system leakage capacitance, reference total differential current, reference leakage current spectrum is performed by filtering the measured values.

By means of a filtering, for example by a moving averaging, a continuous, slow adaptation of the reference value to changing system conditions is achieved. Abrupt changes of the current measured value compared to the thus determined sluggish reference value are recognized as erroneous events (protective conductor demolition).

In implementation of the method according to claim 1 of the invention, the object for a grounded power supply system with an inverter system connected to a subsystem is achieved by an inventive electrical protection device according to claim 6.

For this purpose, the electrical protection device has a device for measuring a leakage current, a computing unit for calculating a leakage current spectrum and an evaluation process unit for testing the calculated leakage current spectra and for signaling the interruption of the protective conductor connection.

The device for measuring a leakage current detects a leakage current specific for the converter system for different power operating points of the converter system. The detection takes place as a reference leakage current in a fault-free state of the grounded power supply system and as a current leakage current during operation of the converter system.

In the arithmetic unit is a transformation of the time course of the leakage current in a Ableitstromspektrum, based on the The evaluation process unit checks whether there is a deviation of the currently calculated leakage current spectrum from the corresponding reference leakage current spectrum.

Furthermore, the device for measuring the leakage current, the arithmetic unit for calculating the leakage current spectrum and the evaluation process unit form an integrated combined device based on a device for determining the network quality as a structural unit.

An integrated combination device offers the advantages already mentioned above of a simplified startup and increased reliability of the electrical protection device and reduces the circuitry complexity. Preferably, an existing device for determining the power quality (PQ device) provides the basis for the integrated combination device.

With the method according to the invention and its implementation by means of the corresponding electrical protection device, an effective monitoring of the protective conductor connection becomes possible. In particular, an interruption of a protective conductor connection to a subsystem can be detected in advance, so that there is no time-consuming and cost-causing interruption of operation.

Fig. 1:

eine Darstellung einer Schutzerdung in einem ungeerdeten Stromversorgungssystem mit einer elektrischen Schutzvorrichtung und

Fig. 2:

eine Darstellung einer Schutzerdung in einem geerdeten Stromversorgungssystem mit einer erfindungsgemäßen elektrischen Schutzvorrichtung.

Further advantageous design features will become apparent from the following description and the drawings, which illustrate a preferred embodiment of the invention with reference to examples. Show it:

Fig. 1:

a representation of a protective ground in an ungrounded power supply system with an electrical protection device and

Fig. 2:

a representation of a protective ground in a grounded power supply system with an electrical protection device according to the invention.

Fig. 1 shows a protective ground in an unearthed (IT) three-phase (AC) power supply system 2 with the active conductors L1, L2, L3. The power supply system 2 comprises a disconnectable subsystem 4 with a resource 6, which is connected via a leading to the subsystem 4 protective conductor connection 8 with a grounding system. All active parts of the ungrounded power supply system 2 are by definition separated from earth 10. The power supply system 2 is further characterized by the system leakage capacitances Cnl of a main system and the system leakage capacitances Cn2 of the subsystem 4, wherein the sum of the system leakage capacitances Cnl and Cn2 due to their parallel connection results in a total network leakage capacitance of the ungrounded power supply system 2. The leakage currents Ia1 and Ia2, which in the present case of the AC system are proportional to the respective system leakage capacitances Cn1 and Cn2, flow via the system leakage capacitances Cn1 and Cn2 (in the faultless state and when the subsystem 4 is switched on).

An insulation monitoring device 12 connected between the active conductors L1, L2, L3 and earth 10 monitors an insulation resistance Riso of the power supply system 2 (in a simplified representation, only the insulation resistance Riso of the main system is shown here, but also parallel connected, not shown, insulation resistances of existing subsystems) ,

In the illustrated operating case, an interruption 20 (first fault) of the protective conductor connection 8 to the subsystem 4 has occurred. If a second fault Rf now occurs in the operating means 6 connected to the subsystem 4 (two-fault situation), for example because of faulty insulation, then a fault current If flows via the touching person and the system leakage capacitance Cn1. Particularly in large-scale power supply systems 2 with large system leakage capacitances Cnl, the fault current If can assume dangerously high values.

An electrical protection device 30 for detecting the interruption 20 of the protective conductor connection 8 has measuring devices 32, 34 and an evaluation process unit 36. In detail, these are a device 32 for measuring the total system discharge capacity of the ungrounded power supply system 2, a device 34 for measuring the total power received via the ungrounded power supply system 2, and an evaluation process unit 36 for testing the measured total system discharge capacity and the measured total power consumed and for signaling the interruption 20 of the protective conductor connection 8.

The apparatus 32 forms, for the measurement of the total system leakage capacitance together with the insulation monitoring device 12, an expanded insulation monitoring device 38, which in turn together with the device 34 for measuring the total power and the evaluation process unit 36 forms an integrated combination device 31 as a structural unit.

In Fig. 2 is the same operation case (two-fault situation) as for the ungrounded power supply system 2 off Fig. 1 for a grounded (TN) three-phase (AC) power supply system 3 with the active conductors L1, L2, L3. In contrast to the ungrounded power supply system 2 ( Fig. 1 ), the grounded power supply system 3 at its feed point to a direct ground connection 9. Due to the interruption 20 of the protective conductor connection 8, the fault current If flows completely across the person touching.

By measuring the total differential current and the total power of the grounded power supply system 3, an interruption 20 of the protective conductor connection 8 is detected in conjunction with an evaluation of the measurement results.

For this purpose, the electrical protection device 40 has a device 42 for measuring a total differential current of the grounded power supply system 3, a device 44 for measuring a grounded via the Power system recorded total power and an evaluation process unit 46 for testing the measured total differential current and the measured total recorded power and for signaling the interruption 20 of the protective conductor connection 8 on.

The total differential current measuring apparatus 42, the total power consumption measuring apparatus 44, and the evaluation processing unit 46 constitute an integrated combination apparatus 41 based on a network quality determination apparatus as a structural unit.

In the case of an inverter system connected to the subsystem - in Fig. 2 If the operating medium 6 can be regarded as such an inverter system, an electrical protective device 50 for detecting an interruption of a protective conductor connection during converter operation is arranged in the supply line of the subsystem 4.

The electrical protection device 50 according to the invention for subsystems 4 with converter system comprises a device 52 for measuring a leakage current, a computing unit 54 for calculating a leakage current spectrum and an evaluation process unit 56 for testing the calculated leakage current spectra and for signaling the interruption 20 of the protective conductor connection 8. The Implementation of the electrical protection device 50 in the form of an integrated combi device 51 on the basis of a device for determining the power quality as a structural unit done.

Unless protective conductor connections to other subsystems are to be monitored, the electrical protection device 40 not provided for converter operation can be dispensed with.

Claims (7)

1. A method for detecting a disconnection (20) of a protective conductor connection (8) with a subsystem (4) of a grounded power supply system (3), comprising a converter system connected to the subsystem (4), comprising the method steps to be executed during activation of the converter system:

   - measuring a leakage current specific to the converter system for each one of different power operating points of the converter system in a fault-free state of the grounded power supply system (3),

   - calculating a reference leakage current spectrum of the respective leakage current specific to the converter system,

   and comprising the method steps to be repeatedly executed during operation of the converter system:

   - measuring a current leakage current for each of the different power operating points of the converter system,

   - calculating a current leakage-current spectrum of the respective current leakage current,

   - testing whether the currently calculated leakage current spectrum deviates from the corresponding reference leakage current spectrum,

   - signaling that there is a disconnection (20) of the protective conductor connection (8) if the test reveals that the currently calculated leakage current spectrum deviates from the corresponding reference leakage current spectrum.
2. The method according to claim 1,
   characterized in that
   calculating the reference leakage current spectrum specific to the converter system and calculating the current leakage current spectrum during operation of the converter system comprises calculating significant leakage-current spectral components, the significant leakage-current spectral components being assigned to the power operating points in a learning phase during activation of the converter system.
3. The method according to claim 2,
   characterized in that
   the currently calculated leakage current spectra are each tested by a comparison at points of the significant leakage-current spectral components.
4. The method according to any one of claims 1 to 3,
   characterized in that
   the reference leakage current spectrum is determined when the power supply system (2, 3) is activated for the first time and immediately after a repeat test of the power supply system (2, 3).
5. The method according to any one of claims 1 to 4,
   characterized in that
   the reference leakage current spectrum is determined by filtering the measured values.
6. An electrical protection device (50) for detecting a disconnection (20) of a protective conductor connection (8) with a subsystem (4) of a grounded power supply system (3), comprising a converter system connected to the subsystem (4), the electrical protection device (50) comprising
   a device (52) for measuring a leakage current, a calculating unit (54) for calculating a leakage current spectrum and an evaluating process unit (56) for testing the calculated leakage current spectra and for signaling the disconnection (20) of the protective conductor connection (8)
   characterized in that
   the evaluating process unit (56) being configured so as to carry out the process steps according to claim 1.
7. The electrical protection device (50) according to claim 6,
   characterized in that
   the device (52) for measuring the leakage current, the calculating unit (54) for calculating the leakage current spectrum and the evaluating process unit (56) form an integrated combined device (51) as a structural unit on the basis of a device for determining network quality.

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